1. Field of the Invention
The present invention relates to methods and apparatus for optically writing and/or reading information on optical discs and more specifically to improvements whereby focus and tracking of a write/read light spot on the information surface of the optical disc is controlled by adjusting movements of the write/read light source.
2. Description of the Prior Art
The currently preferred optical disc technology employs disc elements with spiral or concentric tracks of minute (usually on the order of a micron or less in size), optically-detectable marks. One real-time mode of recording (writing) such marks is by scanning the disc with a small focused spot of radiation (e.g., from a laser) which is modulated "off or on" according to an electrical signal representative of the information to be written. Information is recovered (read) by scanning the tracks with an unmodulated, equivalently small, tightly focused light spot. The recovered information is in the form of a fluctuating electrical signal obtained from a photodetector that senses the modulated read-out light reflected from the recorded disc.
In order to write and read information in the form of such minute markings, optical systems of relatively high numerical aperture ( e.g. N.A.&gt;0.3) are used to focus light to equivalently minute spots. Such optical systems have extremely small depths of focus and the maintenance of proper spot-focus on the disc surface is difficult.
One approach has been to utilize highly-precise turntables for supporting and rotating the optical discs in a predetermined plane. Such apparatus is expensive, but even with such costly turntables it is usually necessary also to provide complex focus-servo devices which effect lens adjustment in response to minute variations in the position of the record surface of the optical disc relative to the focusing lens. Such record surface variations are caused by thickness variations and non-flatness of the disc or turntable surface or apparatus vibrations. Since the discs rotate at high speeds (e.g., 1800 RPM), these focus-servo devices must respond at high frequencies (e.g., 500 Hz. for discs with ground glass substrates and in the range of 1000 to 3000 Hz. for discs with molded plastic substrates). The high numerical aperture lenses present a relatively large mass that presents additional difficulty in effecting requisite focusing accelerations and decelerations of the lens. Therefore these focus-servo devices are also costly, and fragile. U.S. Pat. No. 4,193,091 discloses one example of such a focus control system wherein error signals to dictate adjusting movements of the focusing lens are provided by a focus detection system including a light source which oscillates at a predetermined frequency.
One approach for avoiding adjusting movement of the focusing lens element is described in U.S. Pat. Nos. 3,673,412 and 4,051,529. In the focus control systems of these patents, the image conjugate of the focusing lens is adjusted by means of a movable mirror, which forms a part of the optical path and is adjusted along the optical axis in response to focus error signals. These systems introduce additional optical elements along the light path, which present additional alignment difficulties and decrease the light power available at the disc surface. Such power decrease is undesirable, particularaly for recording at high speeds where the availability of power is quite critical. U.S. Pat. Nos. 4,037,929 and 4,124,273 disclose similar approaches, using electro-optic modulators, for adjusting the image conjugate of the focusing lens to effect focus control. These systems likewise introduce optical inefficiencies and resultant decreases in the recording power available at the recording surface.
U.S. application Ser. No. 160,769, entitled "Improved Optical Disc Method, Media and Apparatus for Writing and/or Reading Information", filed June 18, 1980, in the names of Geyer and Howe provides an alternative approach to the focus control problem. The Geyer and Howe approach flys flexible optical discs, in a dynamically stabilized condition, specifically avoiding localized constraint structures which force the disc to any predetermined position relative to the working lens means. This technique substantially stabilizes the flying disc's neutral plane (the plane within a disc along which material is neither compressed nor extended during bending) in a plane of rotational equilibrium with an unexpected degree of precision, which dramatically reduces focus control requirements. However, some focus imprecisions still exist, e.g. to the extent of thickness variations in the disc.
I have found, however, that with the decreased variation in record surface position (e.g. .ltoreq..+-.5.mu.) existing in the Geyer and Howe flying flexible disc system, a different and highly advantageous focus control technique can be utilized with excellent results.